1. Field of the Invention
The present invention pertains to gate valves, and in particular to gate valves of relatively large size, up to several feet in diameter.
2. Brief Description of the Prior Art
Prior art gate dampers or valves have been employed to provide sealing for conduit systems of relatively large effective flow diameters, typically ranging from one to six feet. One particular type of gate damper employs a flexible plate brought into and out of contact with a discharge outlet of a valve. The gate is typically held in place against the discharge orifice by upstream pressures. The flexible gate, carried by a rigid bar into and out of contact with the discharge outlet, is spaced slightly upstream of the discharge outlet throughout much of its travel. The box has wedge-shaped camming surfaces on either side of the flexible plate. As the flexible plate is brought into contact with the discharge outlet, the wedging surfaces of its bar-like carrier force the flexible plate into contact with the discharge outlet with a wedging action. Thereafter, upstream pressure is applied against the flexible plate, aiding in sealing engagement between the flexible plate and discharge outlet.
When employed in some commercial applications, it is essential that the gate damper provide absolute isolation between two fluid or gaseous components. Accordingly, the gate dampers of the above-described type have been adapted to provide a double sealing in a valve body. The double sealing arrangement was provided in a valve body having an inlet as well as a discharge outlet. Basically, the flexible gate arrangement described above was duplicated to leave a mirror image facing the inlet orifice. As an option, the space between the mirror-image flexible gates was enclosed to provide a purging chamber. However, the purging chamber was relatively large, covering not only the entire area between the pressure gates, but also the area between their rigid, wedging bar-like carriers, and the tracks on which the gates ride, as well as the area of the actuating system used to draw the gates along their tracks, into and out of their sealing positions. The volume of the purging chamber is, therefore, typically quite large, requiring a purging pump of relatively large capacity and a fairly large number of joints or seams which must be maintained free of possible leakage.